1. Field of the Invention
The present invention concerns a microwave load in small-length oversized waveguide form.
2. Description of the Prior Art
The technology of millimetric or sub-millimetric high power waves is currently being developed through generators and amplifiers such as gyrotrons etc. The waveguides used are oversized so that the necessary power can be transmitted. The waveguides, which are generally circular sectioned, have diameters of more than three times and, sometimes, of more than twenty times the wavelength in the infinite free space of the guided wave.
These overdimensioned guides enable a reduction in transmission losses. This is why propagation modes producing low losses are chosen. In the case of circular waveguides, the low loss modes are of the TE.sub.On type (n being a whole number greater than or equal to one) and the mode TE.sub.01 is frequently used.
In order to dissipate a part of or all the power brought into play during tests, for example, use is made of devices called matched loads which are often waveguide elements with high transmission losses, where the power gets dissipated in an absorbent material.
Owing to low transmission losses related, firstly, to the oversizing and, secondly, to the low loss mode chosen, standard loads have a very great absorbent length.
Furthermore, the loads have to absorb not only the power contained in the preferred mode but also the power contained in the unwanted modes made inevitable by the size of the overdimensioned waveguide.
If a portion of the power is not absorbed in the load, there is a risk that it will be reflected towards the oversized waveguide or generator, and this may lead to their destruction.
Existing matched loads generally consist of a waveguide made of an absorbent material which may be closed by a short-circuit at one of its ends. The other end is open, for it is by this end that the waves to be attenuated penetrate. They get propagated firstly in a overdimensioned waveguide which is connected to the input of the load. In the case of a circular waveguide, the diameter of the guide forming the load is substantially equal to or greater than that of the waveguide propagating the waves to be absorbed. The incident waves that have penetrated the load and have not been absorbed are reflected towards the input by the short-circuit and may be absorbed on their return.
The lengths of such loads for the circular mode TE.sub.01 are very great.
At 100 GHz, a load formed by a tube with an internal diameter of 63.5 millimeters will have a length of 7 meters and, at 8 GHz, a load formed by a tube with an internal diameter of 114 millimeters will have a length of 2.50 meters.
One idea proposed to reduce these lengths was to gradually reduce the cross section of the waveguide forming the load in its rear part. The first part of the load, close to the input of the waves to be attenuated, has a constant cross section. It attenuates waves for which the high order modes have high losses and which cannot get .propagated in the zone with reduced section. The second part of the load with a gradually reduced section attenuates the low order modes which have low losses. The lengths of such loads are reduced. For example, at 100 GHz, the above-mentioned load will have a length of 3 meters and at 8 GHz, its length will be only 1.50 meters.
However, this load is accompanied by a major reduction in the maximum level of absorbable power. This reduction in performance levels varies in a ratio of 2 to 5, depending on the degree of initial oversizing. For, the increase in losses consequent to a reduction in the cross-section of the load is significant only if the reduction in the section is great. This means that a high power density gets collected in that part of the load having a greatly reduced section, and that there are risks of breakdown.
Furthermore, each absorbent material dissipates a certain quantity of power per unit of area, and this limits the absorbable power in the part of the load having a greatly reduced section.
The making of a structure with a greatly reduced section is particularly expensive.
There is another known type of load in a reduced-length waveguide form. A load in waveguide form, with a constant section, is taken and a conical or pyramid-shaped metallic element is placed inside the guide, in its rear part. This load has the same drawbacks as above and, moreover, the metallic element has a limited length for mechanical and thermal reasons.
The present invention proposes a microwave load in reduced-length, oversized waveguide form, enabling the absorption of all the power transmitted by an overdimensioned waveguide placed at its input. The costs of making a load such as this are low.